The present invention relates to a method of fabricating a radiation detector comprising a photosensitive sensor associated with a radiation converter. The fields of application of this type of detector are notably the detection of X-rays used in radiology: radiography, fluoroscopy and mammography, and also for nondestructive testing. The invention will be described in relation to an X-ray detector. Of course, the invention may be implemented with any type of detector for which the photosensitive sensor is not directly sensitive to the radiation to be detected, and for which it is therefore necessary to interpose a radiation converter between an entry window of the detector and the photosensitive sensor.
Such radiation detectors are known for example from French patent FR 2 605 166, in which a sensor formed from amorphous silicon photodiodes is associated with a radiation converter.
The operation and the structure of such a radiation detector will now be briefly reviewed.
The photosensitive sensor is generally fabricated from solid-state photosensitive elements arranged in a matrix. The photosensitive elements are fabricated from semiconductor materials, usually single-crystal silicon in the case of CCD or CMOS sensors, polycrystalline silicon or amorphous silicon. A photosensitive element comprises at least one photodiode, at least one phototransistor or at least one photoresistor. These elements are deposited on a substrate, generally a glass plate.
In general, these elements are not directly sensitive to radiation of very short wavelength, such as X-rays or gamma rays. This is why the photosensitive sensor is associated with a radiation converter, which comprises a layer of a scintillating substance. This substance has the property, when it is excited by such radiation, of emitting radiation of longer wavelength, for example visible or near-visible light, to which the sensor is sensitive. The light emitted by the radiation converter illuminates the photosensitive elements of the sensor, which perform a photoelectric conversion and deliver electrical signals that can be exploited by appropriate circuits. The radiation converter will be called a scintillator in the rest of the description.
Certain scintillating substances of the family of alkali metal halides or rare-earth metal on/sulfides are frequently employed for their good performance.
Among alkali metal halides, cesium iodide doped with sodium or with thallium, depending on whether it is desired for the emission to be at around 400 nanometers or around 550 nanometers respectively, is known for its strong X-ray absorption and for its excellent fluorescence yield. It takes the form of fine needles which are grown on a support. These needles are approximately perpendicular to this support and they partly confine the light emitted toward the sensor. Their fineness determines the resolution of the detector. Lanthanum oxysulfide and gadolinium oxysulfide are also widely used for the same reasons.
However, some of these scintillating substances have the drawback of being not very stable—they partially decompose when exposed to moisture and their decomposition releases chemical species that migrate either toward the sensor or away from the sensor, these species being highly corrosive. Notably, cesium iodide and lanthanum oxysulfide have this drawback.
As regards cesium iodide, its decomposition gives cesium hydroxide Cs+OH− and free iodine I2, which can then combine with iodide ions to give the complex I3−.
As regards lanthanum oxysulfide, its decomposition gives hydrogen sulfide H2S, which is chemically very aggressive.
Moisture is extremely difficult to eliminate. The ambient air, in which the sealing operation is carried out, always contains moisture.
One of the important aspects when producing these detectors is to minimize the amount of moisture initially present within the detector, and in contact with the scintillator, and to prevent said moisture from diffusing into the sensor during its operation.
In a first configuration, called an “added-scintillator” configuration, the scintillating substance is deposited on a support through which the radiation to be detected has to pass before reaching the sensor. The assembly is then bonded to the sensor.
In a second configuration, called a “direct-deposition” configuration, the sensor serves as support for the scintillating substance, which is therefore in direct and intimate contact with the sensor. The scintillating substance is then covered with a protective coating. The two configurations each have advantages and disadvantages.
One advantage of the first, attached-scintillator, configuration is that the sensor and the scintillator are joined together only if they have been successfully tested, thereby improving the overall fabrication yield.
Other advantages of this configuration will be apparent on reading French patent application FR 2 831 671.
The aim of the invention is to improve the production of a large detector requiring the attachment of several smaller photosensitive sensors, the radiation detector being fabricated in the first configuration.
FIGS. 1a to 1d describe an operating method for producing attached assemblies.
Small sensors 1 are aligned and positioned individually. A seal 2 is positioned between the sensors 1 and then a liquid adhesive 3 is spread over the sensors 1. The seal 2 prevents the adhesive 3 from flowing between the sensors 1. A substrate 4 common to the various sensors 1 is then bonded to the sensors 1 by means of the adhesive 3. The material of the seal 2 must have a high viscosity so as to prevent it from flowing between the sensors 1. However, this viscosity must be low enough for it to perfectly match a portion of the space that the adhesive 2 must fill between the sensors 1.
As for the material of the adhesive 3, this must have a viscosity low enough to migrate over all the surfaces of the sensors 1 in order to be bonded to the entire surface. However, too fluid an adhesive 3 would have a tendency to extend beyond the periphery of the sensors 1.
With this operating method, it is impossible to develop a single adhesive acting both as adhesive 2 and as adhesive 3. Such a single adhesive would have to reconcile the contradiction of being both fluid enough to migrate over all the surfaces of the sensors 1, and achieve complete bonding, and viscous enough to prevent it from flowing between the sensors 1.
In a radiation detector embodiment, the following operation consists in depositing, on the attached assembly thus fabricated, a layer of liquid adhesive 5 intended to bond a scintillator 6 to the attached assembly. The adhesive 5 is a material that has to be optically transparent so as not to absorb the light emitted by the X-ray-illuminated scintillator 6. The material of the adhesive 5 must have a viscosity low enough for it to penetrate between the sensors 1 and to completely fill the residual space between the sensors 1 left free by the adhesive 2 deposited during the preceding operation. It must also have a viscosity high enough for it not to flow excessively over the periphery of the sensors 1.
The adhesive 5 must be thin enough to allow good image quality. This is because the light generated by the scintillator 6 must pass through this adhesive layer before being absorbed by the sensors 1. This light will be scattered less the thinner the adhesive 5.
For example, liquid adhesives 5 have been used from the families of silicone, epoxy and acrylic adhesives, or any other family of adhesives. These liquid adhesives must be deposited beforehand on one of the two surfaces to be bonded, or on both of them. This adhesive dispensing must be very uniform in terms of thickness and this thickness must be perfectly controlled so as to achieve image quality, and notably resolution, which is uniform at all points on this image.
FIGS. 2a to 2d describe four types of defects encountered when producing the detectors. These fabrication defects result in defects in the image produced by the detector when irradiated with X-rays.
In FIG. 2a, the first defect is characteristic of too low a viscosity of the adhesive 2, which completely interfered in the space available between the sensors 1 and comes into direct contact with the scintillator 6 when the latter is bonded by means of the adhesive 5. This first fabrication defect results in a defect in the image obtained by the detector. The amount of light reaching the sensors 1 will be too great at the junction between the sensors 1, the sensitivity of the detector in these places being too high.
In FIG. 2b, the second defect is characteristic either of too high a viscosity of the adhesive 2 or of too high a viscosity of the adhesive 5. These two adhesives cannot come into contact with each other, so that an air-filled cavity persists between the sensors 1. This may result in a break in the optical continuity of the light paths at the interfaces when the light coming from the scintillator 6 passes through them. As a result, there is a loss of light, and too dark an image at the location of these air-filled cavities.
This type of defect may evolve and create the third defect, shown in FIG. 2c, in which the volume of trapped air may expand and create delaminations at the bonded interfaces.
The fourth defect shown in FIG. 2d may be observed when the sensors 1 have not been correctly aligned in the horizontal plane—there is thus a height discrepancy. A mechanical step when passing from one sensor 1 to its neighbor may result in a break in continuity of the adhesive 5. This type of defect could be resolved by an adhesive 5 having a viscosity low enough to follow the break of the mechanical step. However, too low a viscosity of the adhesive 5 may result in poor lifetime of the optical contact at the step break.
Thus, the four fabrication defects described above always result in breaks in the continuity of the adhesive 5 and absences of material or the presence of air cavities that will result in faults in optically coupling the scintillator 6. At the place of these faults, the image may be exaggeratedly bright (first defect) or exaggeratedly dark (second, third and fourth defects).
Resolution of these defects is faced with having to have the viscosity of the adhesives 2, 3 and 5 which require contradictory properties. It is in fact required of the adhesives 2, 3 and 5 to have sufficiently low viscosities or sufficiently high viscosities for fulfilling contradictory functions.